When I'm traveling at around 30mph, and the gas pedal is depressed enough to keep the speed, the truck starts to buck. I can faintly feel it at high speeds. The engine even does it when the truck is in park and I gradually give it gas. I have replaced the fuel pump, fuel filter, plugs, wires, cap and rotor, I checked the coil and the ecm, map sensor, throttle position sensor, the sensor that sits on the side of the TBI. The timing is correct. I am stumped. Any help would be greatly appreaciated. Thank you.